Atoms' Quantum Spin Controlled in Odd Chilled Gas

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Physicists have revealed a new way to control the spins of atoms,
an achievement that could open the way for new kinds of sensors
while also shedding light on fundamental physics.

While scientists have been able to nudge the
spins of atoms in the past, this new achievement, detailed in
the June 6 issue of the journal Nature, is the first time they've
done it in a strange chilled gas called
a Bose-Einstein condensate.

The researchers say the finding may also be a step toward
spintronics, or electronic circuits that use an electron's
spin instead of its charge to carry information.

Chilled rubidium

The research team, from the Joint Quantum Institute, the National
Institute for Standards and Technology (NIST) and the University
of Maryland, used several lasers to trap rubidium atoms in a
vacuum chamber. The rubidium atoms were in a tiny cloud, about 10
micrometers on a side, where 1 micrometer is about the size of a
bacterium. The atoms were cooled to a few billionths of a degree
above absolute zero. [ Wacky
Physics: The Coolest Little Particles in Nature ]

By chilling the atoms, the researchers created a Bose-Einstein
condensate, a special kind of gas in which all the atoms are in
the same quantum mechanical state, meaning they all had either
"up" or "down" spins; the condensate revealed phenomena that
could ordinarily only be seen at the atomic scale.

In addition, very cold atoms are easier to track, since they are
moving relatively slowly. At normal temperatures, the atoms move
quickly and the apparatus has to be bigger. "You want to give
yourself the time that
ultracold atoms give you," said study researcher Ian
Spielman, a NIST physicist. "And you can do the whole thing in
less space."

The researchers then used another set of lasers to gently push
the cold gas. That small push moved the atoms just enough that
the team could see the atoms with different spins, or magnetic
alignments, move to one side or the other, depending on whether
they were spinning up or down.

The movement is called the spin Hall effect. It involves
particles of different spins moving to one side or the other of a
piece of material when an electric current runs through it. The
particles — they can be electrons or atoms — move perpendicular
to the direction of the current.

Spin Hall effects have been detected before in semiconductors,
but this is the first time an experiment has been done with a
Bose-Einstein condensate.

By inducing this effect in the rubidium, the NIST team showed
they could control where the atoms of different spins went, in
this case by applying a laser.

Spintronics and sensors

The concept has other applications, one of which is
"spintronics." A spintronic computer would be able to store more
data and complete calculations more quickly than traditional
computers.

While no one has come close to developing spintronic circuits,
closer on the horizon are better inertial sensors, which detect
acceleration and motion. Sophisticated ones are used in physics
experiments, Spielman said. For instance, watching the atoms of
different spins drift can show small variations in gravitational
and magnetic fields.

As to
fundamental physics, the experiment showed that since it's
possible to induce the spin Hall effect, then it's also possible
to induce its quantum-mechanical cousin. The quantum spin Hall
effect describes a state of matter that exists in two-dimensional
materials that are usually insulators rather than electrical
conductors. It involves spin-up particles — usually electrons —
conducting along one edge of the insulator, while spin-down
electrons travel along the other side. That will be explored in a
future set of experiments, Spielman said.

Matthew Beeler, lead author and now a staff scientist at the
Johns Hopkins University Applied Physics Laboratory, said the
experiment expands the "toolbox" available to scientists who want
to manipulate particles and their spins. "The power is the
ability to combine the tools in new ways. You could take spin
Hall effect and add it to something else," Beeler said.